Schmitz, S. I. (Corresponding author)FZJ*

2002
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich

Report No.: Juel-4006

Abstract: Due to their electrical properties electroceramic materials (e.g. Titanates) have a great potential for applications in microelectronics. Apart from the use in sensors, actuators and electro-optical elements, thin films consisting of high permittivity materials are of great interest as dielectrics for semiconductor memories such as DRAMs. Both the integration of these materials into the manufacturing process of conventional CMOS devices as well as the structural and electrical characterisation are therefore topics of research. The use of electroceramic materials with much larger dielectric constants than those of SiO$_{2}$/Si$_{3}$N$_{4}$ and TaO$_{5}$, dielectric materials in present DRAMs, allows the design of memory cells in a simple stack structure with an increased memory density in the Gb range. However, these materials are not ideal insulators, suffer from charge losses in the cell capacitor due to leakage and relaxation currents. In order to retain the information, the charge has to be renewed in regular time intervals (refresh). Therefore, the dielectric constant and the leakage current are important characteristics of electroceramic thin films for their use in future DRAM applications. The main part of this thesis considers the electrical characterisation of SrTiO$_{3}$ and (Ba$_{0,7}$Sr$_{0,3}$)TiO$_{3}$ thin films deposited by chemical solution deposition (CSD) and metal organic chemical vapour deposition (MOCVD). Apart from the capacitance measurements the focus lies an the investigation of the leakage current. An automated probe station with a resolution of less than 100 fA was constructed and built to measure leakage currents . These measurements were performed an metal-insulator-metal (MIM) capacitors having different dielectric film thicknesses and different materials were used as top electrode. The test temperature varied between room temperature and 475 K. The current transport through a MIM-structure was simulated using the finite difference calculation method. Based an the thickness dependence of the permittivity, interface layers with a largely reduced dielectric constant compared to the bulk were assumed at the electrode interfaces. Introduction of these layers, also called "dead" layers, and the consideration of the thin film properties (charge carrier mobility and doping) and the barrier lowering due to the Schottky effect resulted in a good agreement between the Simulation and experimental results. In particular, the surprising experimental thickness dependence - decreasing leakage current with decreasing thickness - could be explained. Over a wide voltage range the measurements as well as the simulated leakage currents behave qualitatively like the thermionic emission (Schottky emission). However the absolute value of the leakage current is smaller than the Schottky emission current and, therefore, is bulk limited. The conductivity of the electroceramic thin film depends an the electron concentration within the film which in turn is strongly influenced by the electron concentration at the interface. This influence of the interface explains the Schottky-like behaviour of the leakage current as well as its dependencies an different electrode materials.

Keyword(s): titanate ; electrical properties

1. Elektronische Materialien (IFF-6)